Frontrunning: April 6

"Mr. Iksil has done so much bullish trading that he has helped move the index, traders say. Now, even as Mr. Iksil is selling credit protection on the company index, a number of hedge funds and other investors are buying protection on it."

Some investors say they are betting that Mr. Iksil could have to exit some of his bullish trades, perhaps because the pending Volcker rule limiting bank risk-taking would push up the cost of credit protection. J.P. Morgan has said the Volcker rule doesn't prohibit its CIO unit from investing or hedging activities.

One person familiar with the matter said the bank has run tests that show Mr. Iksil's positions likely will be profitable in any economic or market downturn.

A sign of how hot the trade is: The net "notional" volume in the index ballooned to $144.6 billion on March 30 from $92.6 billion at the start of the year, according to Depository Trust & Clearing Corp. data.